Sterile water: single use by design
Sterile water for injection contains no preservative. The moment a needle pierces the stopper, the clock starts: the contents are no longer protected, and anything introduced on entry can grow freely in pure water with nothing to stop it.
That is why sterile water is labelled single use. It is not a liability disclaimer, it is the design. Once opened, it is not suitable for reconstitution work that needs a second entry, and it is not suitable for keeping. Use what you need and discard the rest.
Reaching for sterile water across multiple entries is one of the most common contamination routes in small-scale research setups, precisely because the packaging looks the same as preserved product.
What the benzyl alcohol is for
Bacteriostatic water is sterile water with one addition: 0.9% benzyl alcohol, a bacteriostatic preservative. It does not sterilise anything and it does not clean a dirty entry. What it does is inhibit bacterial growth inside the vial between entries.
That single property changes the product's entire use profile. A multi-dose vial can be entered repeatedly over weeks, refrigerated between uses, and remain a valid diluent. Without the preservative, none of that is possible.
The preservative is a safety net, not a substitute for technique. Wipe the septum, let the alcohol pad dry, use fresh sterile consumables. The preservative covers what technique misses; it does not forgive what technique ignores.
When sterile water is still the right call
For applications where any preservative is unacceptable, sterile water remains correct: draw what you need, use it once, discard the rest. The mistake is not choosing sterile water, it is asking it to do a multi-entry job it was never designed for.
Which to buy
Multi-entry work needs bacteriostatic water. BuyBac supplies it in three formats: 10mL single-entry ampoules at $5, 30mL multi-dose vials at $30, and the 5x10mL bench pack at $20. Batch documented on request, dispatched from Perth, $15 flat shipping Australia wide.